Wet development in electrophotography is generally carried out by developing an electrostatic latent image on an electrophotographic photoreceptor with a liquid developer comprising a dispersing medium, usually an aliphatic hydrocarbon, having dispersed therein toner particles mainly comprising a resin and a colorant. The thus formed toner image is transferred to transfer paper and fixed thereon.
Where photosensitive paper or film coated with a photoconductive material, such as lead oxide, is used as a photoreceptor, the transfer process may be omitted, that is, the toner image may be fixed on the photoreceptor.
Wet development is also often employed as a development system for electrostatic recording consisting of forming an electrostatic latent image on a dielectric film by means of electrodes without using a photoreceptor.
The wet development system is primarily based on electrophoresis of fine toner particles of from submicrons to several microns in a carrier liquid having high electrical resistance, such as an aliphatic hydrocarbon. Therefore, this system is characterized by ease of attaining higher resolving power than by dry development system using toner particles of several microns or greater.
According to the early literature produced by K. A. Metcalfe (J. Sci. Instrum., Vol. 32, p. 74 (1955) and ibid, vol. 33, p. 194 (1956)), useful pigments for liquid developers include carbon black, magnesium oxide and other various organic or inorganic pigments, and useful carrier liquids include gasoline, kerosine, and carbon tetrachloride.
The early patent publications by Metcalfe mention usefulness, as a carrier liquid, of halogenated hydrocarbons (JP-B-35-5511, the term "JP-B" as used herein means an "examined published Japanese patent application"), polysiloxanes (JP-B-36-14872), ligroin, and mixtures of these petroleum hydrocarbons (JP-B-38-22343 and JP-B-43-13519).
Patent publications relating to preparation of toners also contain frequent references to a liquid carrier. For example, JP-B-40-19186, JP-B-45-14545, and JP-B-56-9189 describe that aromatic hydrocarbons, such as benzene, toluene and xylene, and aliphatic hydrocarbons, such as n-hexane, isododecane, Isopar G, H, L, M or V (product of Exxon Chemical Corp.), are useful as a carrier liquid, which sometimes serve as a dispersing medium for polymerization.
Because most of these carrier liquids proposed to date are organic solvents of high vapor pressure, there are involved such problems that: (1) they vaporize on fixing, etc., tending to cause environmental pollution; (2) they are flammable; and (3) they remain in transfer paper after fixing and give off a solvent smell.
In order to solve these problems, it has been proposed to decrease the vapor pressure of a carrier liquid by using a hydrocarbon type petroleum solvent of low vapor pressure or a polymerized hydrocarbon which is solid at ambient temperature (see, for example, JP-A-63-167375, JP-A-2-6965, and JP-A-2-6967, the term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, if the molecular weight of a hydrocarbon is increased in an attempt to reduce the vapor pressure, it generally follows that the viscosity of the carrier liquid increases, tending to adversely affect the rate of development. Besides, the melting point of the carrier liquid also increases to approximately room temperature so that development always needs heating, which will cause reduction in copying speed and necessity of much heat energy in the development and fixing parts in actual development operation, and which will lead to waste of energy, thermal pollution, and deterioration of a developer.
Use of a hydrocarbon solution having a resistivity of not less than 10.sup.9 .OMEGA..cndot.cm and a dielectric constant of not more than 3.0 as a carrier liquid is taught in JP-A-51-89428. Like this, conventionally proposed carrier liquids are mostly non-polar hydrocarbon solutions having a high resistivity and a low dielectric constant. It is experimentally known in the art that a carrier liquid whose resistivity is lower than an adequate level breaks a latent image on a photoreceptor or causes a bias leak in the development and transfer parts, failing to obtain a satisfactory image.
Additionally, developers containing a non-polar carrier liquid having an excessively high resistivity and a low dielectric constant have not always produced satisfactory results with respect to toner charging properties and toner charge stability with time. That is, there has been a tendency that the charge quantity of a toner decreases with time or the proportion of a toner charged to the opposite polarity increases.
Hence, under the present situation, a satisfactory carrier liquid for a liquid developer has not yet been developed.